This invention relates to a current sensing circuit, in particular a current sensing circuit for sensing the current through a power semiconductor device such as an insulated gate bipolar transistor (IGBT).
U.S. Pat. No. 4,990,978 and EP-A-390485 describe current sensing circuits each of which has first and second terminals and comprises a power semiconductor device having a main current carrying section consisting of a number of parallel-connected active device cells with first and second main electrodes coupled to the first and second terminals, respectively, of the circuit and a control electrode and a sense current carrying section comprising at least one sense cell similar to the device cells and having a first main electrode coupled to the first terminal and a second main electrode, and a current sensing arrangement comprising a first resistor coupling the second main electrode of the sense current carrying section to the second power supply line.
In these circuits, the current through the sense current carrying section follows that through the main current carrying section and the voltage across the first resistor produced by the current through the sense current carrying section may be compared with a reference voltage provided by for example a band gap reference generator to provide an indication when a given current level is reached. However such circuits present problems in that any variation in the absolute value of the resistance of the first resistor, or variations in temperature which may affect its resistance, will of course affect the voltage sensed for a given current. Accordingly there may be variations between different processing batches or variations due to external effects such as temperature changes.